It is well known to apply an electroless metal plating to an insulating substrate, such as plastic, by depositing a noble metal, such as palladium, gold or platinum, as an initiator or catalyst for the electroless metal plating onto the surface of the insulating substrate, and then dipping the substrate into an electroless metal plating solution. It is also known to catalyze a surface with a non-noble metal catalytic layer, e.g., Cu, by reduction of a copper compound adsorbed on the surface.
Various techniques have been proposed to form a circuit on the insulating substrate by electroless metal plating for the manufacture of printed circuits on flexible or rigid substrates. Among the methods utilized for forming printed circuit boards is the method disclosed in U.S. Pat. No. 3,929,483 wherein a photoresist is laid down on a surface which is highly absorptive to silver salts. Such a surface is anodized aluminum. The photoresist on the surface is then exposed to light through a negative to obtain a desired circuit pattern. The portions not struck by light are removed by washing in a suitable solvent. This leaves a resist image on top of the surface where light exposure has taken place and no resist on the surface where the resist has been removed by washing with the solvent. Thereafter, the substrate is soaked in a concentrated solution of silver nitrate wherein the silver salt is held by the absorptive surface not coated by the resist. This silver salt is then reduced to metallic silver by treatment with a suitable reducing agent. The photoresist is then removed by treatment with an appropriate solvent leaving the desired silver image in the absorptive medium. This silver metal deposit acts as a catalyst for subsequent electroless plating. It should be noted that in accordance with the teachings of this patent, the silver nitrate is held only in those areas where the resist has been washed off and is not absorbed by and does not coat the photoresist layer which is left on the specimen. In essence, the resist layer acts as a poison for the deposition of the silver nitrate.
In another method used in the prior art, a pattern of resist is laid down on an insulating substrate. The entire surface, including both resist and exposed substrate areas, is treated and coated with a sensitizer and an activator, and then the entire surface is treated with a metal deposition solution. Metal deposits both on the substrate and on the resist-covered areas and the resist with its electrolessly deposited metal coating is later removed by means of a solvent to leave only the metal pattern on the substrate, removing the metal pattern that overlies the resist upon removal of the resist. In this method, because of the relatively heavy formation of metal over the resist area, the edges of the conductors have a tendency to be ragged and hence resolution is not as good as required for some uses.
Still another electroless deposition method involves first sensitizing and activating the entire substrate surface before putting down a resist pattern. Then, after putting down the resist pattern, the entire surface is treated with the electroless metal deposition solution. In this method, metal deposits only on the sensitized and activated exposed substrate areas and not on the resist-covered areas. Resolution is quite good in this method, however, this method has serious disadvantages, e.g., (1) in that sensitization and activation of the substrate surface produces a surface which may have a relatively low resistivity between the deposited conductors and if spacing is to be very close, as is required in many of today's applications, this may cause electrical breakdown, and (2) manufacturing handling problems in that the catalytic surface is highly sensitive to contamination, scratching and the like which can result in defective circuits.
More recently, a coworker has suggested a method of manufacturing a printed circuit board by a wholly additive process which results in a pattern of reasonably high resolution and does not result in unwanted lowering of the surface resistance on a substrate between pattern lines. That method comprises applying a removable negative mask onto a surface of the substrate whereby portions of the surface are exposed to form a positive circuit pattern, sensitizing the circuit pattern portions and the negative mask to form thereon a catalytic species capable of catalyzing an electroless metal deposition, removing the negative mask and thereby the catalytic species thereon to delineate the catalytic species and the circuit pattern on the surface and electrolessly depositing a metal onto the delineated catalytic species in the circuit pattern. A preferred variation of this method includes the step, after sensitization of the surface and before removing the mask, of applying a thin flash metal electroless plating of from 0.003 to 0.020 mils in thickness over the catalytic layer. After the formation of this flash plating, the mask is removed together with any catalyst and flash plate overlying it. Finally, the desired metal is then electrolessly deposited onto the remaining portions of the flash plate surface pattern. This flash metal deposit in accordance with that method results in a circuit image that is extremely stable and which can be stored indefinitely prior to electroless plating.
In the aforementioned process, as in many prior art processes, line resolution is limited by the spreading of the line width during buildup of the conductor. This spreading also makes manufacturing more difficult and may limit the line resolution achievable. I have now discovered a modification of this last mentioned process which not only retains the advantages of that process but further improves the resolution and wall definition of the final printed circuit pattern.